X-ray-induced inhibition of mammalian cell growth and proliferation will be studied, with particular emphasis on DNA synthesis in the irradiated and the first prostirradiation generations. Using fiber autoradiography and isopycnic banding, inhibition of synthesis will be analyzed in terms of replicon initiation inhibition in HeLa S3, EMT6, and other cells. Comparisons will be made with the inhibition caused by bromodeoxyuridine photolysis with 313 nm radiation. Caffeine-mediated suppression of the inhibition will be examined further. The relation between inhibition and cell killing will be sought. The effects of various metabolic inhibitors, including caffeine and its analogs, as well as other inhibitors of DNA synthesis, will be examined in irradiated cells, in further study of x-ray-induced potentially lethal damage.